Direct current (DC) architectures are well known, for example for the transmission and distribution of power. DC architectures generally provide efficient (low loss) distribution of electrical power relative to alternating current (AC) architectures.
The importance of DC architectures has increased because of factors including: (1) the reliance of computing and telecommunications equipment on DC input power; (2) the reliance of variable speed AC and DC drives on DC input power; (3) the production of DC power by solar photovoltaic systems, fuel cells, and various wind turbine technologies; (4) propulsion systems in electric and hybrid vehicles, marine applications; (5) aerospace applications; (6) micro-grids and smart grids, including the above, energy storage and electric charging stations; and (7) other systems that require converters with varying input voltage and load.
The widespread use of DC architectures has also expanded the need for DC-DC power converter circuits. Moreover, there is a further need for DC-DC power converter circuits that are efficient and low cost.
Traditionally, cost reduction is achieved in part by (1) reducing the components of DC-DC power converters, and (2) increasing the switching frequency of DC-DC power converters. These cost reduction methods can be achieved by implementing transformerless DC-DC converters that switch at high frequency. High frequency operation allows the circuit designer to reduce the size, and therefore the cost, of expensive components such as transformers, inductors and capacitors. Two of the most common transformerless DC-DC converters are the buck converter 10, as shown in FIG. 1, for stepping down the voltage, and the boost converter 12, as shown in FIG. 2, for stepping up the voltage.
While both of these circuits are capable of achieving very high conversion efficiency when the input-to-output voltage ratio is near unity and the switching frequency is relatively low, their efficiency is less than optimal when the voltage ratio becomes high or the switching frequency is increased to reduce the total size of the converter. In addition, in their basic form they do not provide galvanic isolation. Loss of efficiency, along with other operational problems, are caused by circuit parasitics, including such circuit effects as diode forward voltage drop, switch and diode conduction losses, switching losses, switch capacitances, inductor winding capacitance, and lead and trace inductances. Furthermore, it is known in the prior art that boost converters in particular are susceptible to parasitic effects and high efficiency operation requires low step up ratios, e.g. 1:2 or 1:3.
B. Buti, P. Bartal, I. Nagy, “Resonant boost converter operating above its resonant frequency,” EPE, Dresden, 2005, is an example of a resonant DC-DC power converter, where a resonant tank is excited at its resonant frequency to achieve high step-up/step-down conversion ratios without the use of transformers. An H-bridge based resonant DC-DC power converter was proposed by D. Jovcic (D. Jovcic, “Step-up MW DC-DC converter for MW size applications,” Institute of Engineering Technology, paper IET-2009-407) and modified for enhanced modularity by A. Abbas and P. Lehn (A. Abbas, P. Lehn, “Power electronic circuits for high voltage dc to dc converters,” University of Toronto, Invention disclosure RIS #10001913, 2009 Mar. 31).
The converter disclosed in B. Buti, P. Bartal, I. Nagy, “Resonant boost converter operating above its resonant frequency,” EPE, Dresden, 2005, requires two perfectly, or near to perfectly, matched inductors, each only utilized half of the time, to function properly. Perfect matching is not viable in many applications. Moreover, the fact that the inductor is only utilized half of the time effectively doubles the inductive requirements of the circuit. This is undesirable as the inductor is typically the single most expensive component in the power circuit. Furthermore, the converter in B. Buti, P. Bartal, I. Nagy, “Resonant boost converter operating above its resonant frequency,” EPE, Dresden, 2005, requires both a positive and negative input supply. This is often not available.
The converters disclosed in D. Jovcic, “Step-up MW DC-DC converter for MW size applications,” Institute of Engineering Technology, paper IET-2009-407, and A. Abbas, P. Lehn, “Power electronic circuits for high voltage dc to dc converters,” University of Toronto, Invention disclosure RIS #10001913, 2009 Mar. 31, uses four high voltage reverse blocking switching devices. For medium frequency applications (approx. 20 kHz-100 kHz) such devices are not readily available thus they need to be created out of a series combination of an insulated-gate bipolar transistor (“IGBT”) and a diode, or a metal oxide semiconductor field effect transistor (“MOSFET”) MOSFET and a diode. This not only further increases system cost but it also nearly doubles the device conduction losses of the converter.
Galvanic isolation and larger voltage boost and buck ratios are possible with resonant and quasi-resonant DC-DC converters. These converters use inductive and capacitive components to shape the currents and/or voltages so that the switching losses are reduced allowing higher switching frequencies without a large efficiency penalty as explained in N. Mohan, T. Undeland, W. Robbins, “Power electronics: converters, applications, and design,” Wiley, 1995. Resonant and quasi-resonant DC-DC converters can be implemented with or without galvanic isolation.
A resonant converter with galvanic isolation is found in Bor-Ren Lin and Shin-Feng Wu, “ZVS Resonant Converter With Series-Connected Transformers,” Industrial Electronics, IEEE Transactions on, vol. 58, no. 8, pp. 3547-3554, August 2011. In this work, a series resonant converter is implemented with multiple transformers connected in series. The proposed converter is designed to be used as a power factor pre-regulator in consumer electronic applications. The converter operates near the characteristic frequency defined by the resonant capacitor and resonant inductor. ZVS is achieved for all of the input switching components.
This converter developed by Bor-Ren Lin and Shin-Feng Wu uses a conventional resonant converter design approach. The resonant tank is only able to provide minimal voltage boosting, if necessary, and any voltage boosting or bucking must come entirely from the transformer turns ratio. The small amount of voltage boosting that can be provided is used when the input voltage is low. Furthermore, due to the resonant tank design, this converter would not be suitable to control the power flow between an input and an output voltage source.
Series resonant converters and parallel resonant converters are known to be very efficient for a small range of operating points. They can be implemented without galvanic isolation like the ones in FIGS. 3(a), 3(b) and 3(c) or with galvanic isolation. For applications that require a large range of input voltages and loads, they are not ideal. As shown in B. Yang, “Topology Investigation for Front End DC/DC Power Conversion for Distributed Power System”, Ph.D. Dissertation, Virginia Tech, 2003, both series resonant converters and parallel resonant converters suffer from large circulating currents, and large switching currents when the input voltage is high.
In B. Yang, “Topology Investigation for Front End DC/DC Power Conversion for Distributed Power System”, Ph.D. Dissertation, Virginia Tech, 2003 the author shows that some of the limitations in traditional series resonant or parallel resonant converters can be overcome by using an LLC resonant converter. The isolated LLC converter is shown in FIG. 4(b).
R. L. Lin and C. W. Lin, “Design criteria for resonant tank of LLC dc to dc resonant converter”, IEEE 2010, presents a conventional design approach to obtain an LLC step down converter. The designed converter has a maximum voltage gain from the resonant tank of only 1.44, which is needed when the input voltage is at a minimum. For high input voltage the circuit is operated at, or just below, unity gain. A 9:1 transformer provides the net voltage step down needed for the application.
H. Hu, X. Fang, Q. Zhang, Z. Shen, and I. Batarseh, “Optimal design considerations for a modified LLC converter with wide input voltage range capability suitable for PV applications,” ECCE 2011, is an example of a conventional LLC design methodology applied to a step up converter where the resonant circuit provides close to unity gain. All of the voltage gain is achieved through the output transformer.
In both of the works of R. L. Lin et. al. and H. Hu et. al., the conventional LLC design methodology used yields a resonant tank with very low voltage boosting properties. Furthermore, both designs require a resistive load at the output for proper functionality. These converters, and all LLC converters designed with the conventional method, are not suitable for applications where the power flow between two voltage sources is regulated.
In U.S. Pat. No. 6,344,979 an LLC converter is claimed where the converter is operated between the two characteristic frequencies of the converter,
                    ⁢          ω      =                                    L            r                    ⁢                      C            r                                                  ⁢    and                      ⁢          ω      =                                    (                                          L                m                            +                              L                r                                      )                    ⁢                      C            r                              to maintain output voltage regulation. However, the authors failed to address the high voltage gain region of operation and the advantages of operating there, as well as how, by choosing the right components, the designer can always ensure operation in this region. In addition, the zero current switching region of operation, designated as “LHS Operation” in this document, was not utilized nor were the benefits of operating in this region identified. The “LHS Operation” region is also only usable by a careful selection of resonant tank components, as identified in the current invention.